Lymphatic filariasis and onchocerciasis (river blindness) are deforming parasitic diseases that affect millions of people in the tropics. This proposal addresses the central issues of diagnosis and quantitation of filarial infections. Our laboratory has developed sensitive and specific new tests for several filarial infections based on detection of parasite products in blood (circulating filarial antigens) or detection of human antibodies to recombinant parasite proteins. This proposal describes an ambitious research program to continue and expand this important work. I. CIRCULATING PARASITE ANTIGENS IN HUMAN FILARIASIS: These studies will focus on the use of W. bancrofti antigen detection to study basic issues in filariasis epidemiology with a new multicenter study of infection prevalence, intensity, and disease in bancroftian filariasis and additional studies of amicrofilaremic W. bancrofti infections in "antigen-positive endemic normals." Significant progress has been made in the study of circulating antigens of O. volvulus, B. malayi, and L. loa. Additional studies are needed to develop practically useful antigen assays for these infections. II. RECOMBINANT ANTIGEN-BASED ANTIBODY ASSAYS FOR FILARIASIS: Onchocerciasis studies will test population-based serum sets to determine the value of new assays based on recombinants OC3.6, 9.3, and OVICF for assessing endemicity and for detection of prepatent infections in children. We will also continue studies to develop and validate antibody assays for diagnosis of Brugia infections and clinical lymphatic filariasis in amicrofilaremic persons with negative antigen tests.